Perhaps this already exists and I've not found it... A CSV template that allows for importing a CSV of an estimate from another software application into Zoho Books.

We use a proposal-generating application that offers seamless integration with Quickbooks Online and the ability to export the proposal as a CSV file, which can be imported into Quickbooks desktop & Peachtree.

It will make the decision to move away from QB easier if such as template exists (as I know there is not an integration).